In addition to high breakdown voltage, semiconductor power elements are required to have low ON-state resistance and low switching loss. However, silicon (Si) power elements mainly used for semiconductor power elements at present are approaching their theoretical performance limits. Silicon carbide (SiC) has a dielectric breakdown field strength that is greater than Si by approximately one order of magnitude. For this reason, a thickness of a drift layer maintaining a breakdown voltage is set to be approximately one-tenth thinner than that of the Si power element and an impurity concentration of this drift layer is set to be one hundred times greater than that of the Si power element, so that element resistance can be theoretically reduced by three orders of magnitude or more. In addition, SiC has a bandgap that is approximately three times larger than Si and thus is capable of operating in high temperatures. For this reason, SiC semiconductor elements exceeding the performance of Si semiconductor elements are awaited.
It is known that, in the SiC semiconductor element, a basal plane dislocation (BPD) present in a SiC drift layer expands to form a stacking fault during bipolar operation (Non-Patent Document 1: M. Skowronski and S. Ha, “Degradation of hexagonal silicon-carbide-based bipolar devices” Journal of Applied Physics 99, 011101 (2006)). When electrons and holes are recombined in the BPD, energy of this recombination causes the BPD to expand to form a stacking fault. Since a stacking fault has a high resistance, the element resistance increases as the stacking fault expands. This phenomenon is known as a “bipolar degradation phenomenon”.
Patent Document 1 (Japanese Patent Application Laid-open Publication No. H09-270512) describes an IGBT (Insulated Gate Bipolar Transistor) utilizing a Si (silicon) substrate in which the holes generated in a side diffusion region of a p-type semiconductor layer facing a cell region are removed by flowing to an emitter electrode.